In broadcasting stations for producing and broadcasting television programs, the MPEG (Moving Picture Experts Group) technique is commonly used to compress/encode video data. In particular, in the fields of recording of video data on a randomly accessible recording medium material such as a tape and transmission of video data via a cable or a satellite, the MPEG technique is becoming a de facto standard.
An example of processing carried out in a broadcasting station until a video program created therein is transmitted to each home will be described in brief. First, encoder provided in a camcoder comprising a video camera and a VTR (Video Tape Recorder) integrated together is used to encode and record source video data on a magnetic tape. In this case, the encoder of the camcoder encodes the source video data so as to be suited for a recording format for a VTR tape. For example, an MPEG bit stream recorded on the magnetic tape has a GOP structure in which one GOP is configured by two frames (for example, I, B, I, B, I, B, . . . ). In addition, the MPEG bit streams recorded on the magnetic tape has a bit rate of 18 Mbps.
Next, a main broadcasting station carries out an edition process to edit the video bit stream recorded on the magnetic tape. To achieve this, the GOP structure of the video bit stream recorded on the magnetic tape is converted into one suitable for the edition process. In the GOP structure suited for the edition process, one GOP is configured by one frame and all pictures are of the I type. This is because the I pictures, which have no correlations with other pictures, are most suitable for edition in frames. In an actual operation, the video stream recorded on the magnetic tape is decoded back into baseband video data. This baseband video signal is re-coded so that all pictures are converted into the I type. By carrying out the decoding and re-coding process in this manner, a bit stream having the GOP structure suited for the edition process can be generated.
Next, to transmit the edited video program generated by the above described edition process, from the main station to local stations, the bit stream of the edited video program is converted into a GOP structure and bit rate suitable for a transmission process. For example, in the GOP structure suited for transmission between broadcasting stations, one GOP is configured by 15 frames (for example, I, B, B, P, B, B, P, . . . ). In addition, the bit rate suitable for transmissions between broadcasting stations is preferably high, that is, 50 Mbps or higher because an exclusive line consisting of optical fibers and having a high transmission capacity is commonly provided between broadcasting stations. Specifically, the bit stream of the edited video program is decoded back into the baseband video data. Then, the baseband video data is re-coded to have the GOP structure and bit rate suited for transmissions between broadcasting stations.
In a local station, an edition process is carried out to insert commercial films specific to the locality into the video program transmitted from the main station. That is, as in the above described edition process, the video stream transmitted from the main stream is decoded back into the baseband video data. By re-coding the baseband video signal so that all pictures are converted into the I type, a bit stream having the GOP structure suited for the edition process can be generated.
Subsequently, to transmit the video program edited in the local station to each home via a cable or a satellite, conversion into the GOP structure and bit rate suited for the transmission process is carried out. For example, in the GOP structure suitable for transmissions to each home, one GOP is configured by 15 frames (for example, I, B, B, P, B, B, P, . . . ) and the bit rate suitable for transmissions to each home is low, that is, about 5 Mbps. Specifically, the bit stream of the edited video program is decoded back into the baseband video data. Then, the baseband video data is re-coded into the GOP structure and bit rate suitable for transmissions.
As understood from the above description, the plurality of decoding and encoding processes are repeated while the video program is being transmitted from the broadcasting station to each home. In fact, the broadcasting station requires various signaling processes other than those described above, so that the decoding and encoding processes must be repeated for each signaling process.
As is well known, however, the encoding and decoding processes based on the MPEG standard are not 100% reversible. That is, the baseband video data before encoding is not perfectly the same as the decoded video data and has its image quality degraded due to the encoding and decoding processes. Disadvantageously, repetition of the decoding and encoding processes may progressively degrade the image quality, as described above. In other words, repetition of the decoding/encoding processes may accumulate degradation of the image quality.
The present invention is provided in view of these circumstances, and it is an object of the present invention to provide a transcoding system that can prevent degradation of the image quality even if the decoding and encoding processes are repeated to change the structure of the GOP (Group of Pictures) in the encoded bit stream encoded based on the MPEG standard.